This invention relates to a biodegradable biaxially stretched film.
Most of conventional plastic products, especially plastic packaging materials are discarded soon after use, and their disposal problems are pointed out. Among general purpose packaging plastics, as representative ones, polyethylene, polypropylene, polyethylene terephthalate (xe2x80x9cPETxe2x80x9d), etc. can be cited. But these materials are high in heat buildup when burned and there is a possibility of damaging the incinerator during burning treatment. Further, polyvinyl chlorides, which are large in the amount of use even now, cannot be burned due to their self-extinguishing properties. Also, in many cases, plastic products including such materials which cannot be burned, are buried. But due to their chemical and biological stability, they scarcely decompose but remain, thus causing a problem that they shorten the life of burial sites. Therefore, plastic products that are low in heat buildup during burning, decompose in soil, and safe are desired, and many researches are being made.
As one example, there are polylactic acids. For polylactic acids, the heat buildup during burning is less than half that of polyethylene, and hydrolysis proceeds naturally in soil or water and then they are decomposed by microorganisms into unharmful materials. Now researches are being made for obtaining molded products, specifically film sheets and containers such as bottles using polylatic acids.
Polylactic acid is a polymer formed by condensation-polymerizing a lactic acid. Lactic acids have two kinds of optical isomers, i.e. L-lactic acid and D-lactic acid. Their crystallizability varies with the ratio between these two. For example, a random copolymer of which the L-lactic acid to D-lactic acid ratio is 80:20 to 20:80 has no crystallizability. In other words, it is a transparent, completely amorphous polymer which softens near its glass transition point of 60xc2x0 C. On the other hand, for a homopolymer made up of only L-lactic acid or D-lactic acid, although its glass transition point is likewise 60xc2x0 C., it becomes a semicrystalline polymer having a melting point of 180xc2x0 C. or over. The semicrystalline polylactic acid turns into an amorphous material that excels in transparency, by rapidly cooling after melt extrusion.
It is known that it is possible to improve its strength and shock resistance of a polylactic acid by biaxially stretching it during forming into film. Further, it is known to manufacture a film which will not substantially shrink by suppressing the heat shrinkage of the film by heat-treating it after biaxial stretching. The heat shrinkage rate is determined by the heat treatment temperature and time of the film and the properties of the raw materials used. So the heat treatment temperature and time are suitably adjusted according to the properties of the film material.
After packaging an article in a film, clear fold lines are sometimes formed by slightly fusing the film by applying a hot plate so as not to easily rise. Specifically, this is done for packaging folded neatly in end faces of a cubic article such as video tapes and cassette tapes, packaging of cubic gums, and packaging of boxed tobacco. For them, stretched polypropylene film, cellophane, etc. are used. For them, K-coat film and K-coat cellophane having its surface coated with vinylidene chloride are used. Tacking is usually done by melting the vinylidene chloride layer by pressing a hot plate.
But such vinylidene chlorides are said to be one of today""s environmental pollution sources, and they cause various problems. For example, they promote production of dioxin if burned at low temperature. Thus, for a polylatic acid biaxially stretched film too, it is not preferable to perform K-coating.
To polylactic acid biaxially stretched film, by adjusting the manufacturing method, it is possible to impart heat setting properties while suppressing heat shrinkage. This is because a polylatic acid is low in crystallizability compared with polypropylene or polyethylene terephthalate. Further, by setting it to a film having a suitable crystallizability, it is possible to solve the above problems.
An object of the present invention is to provide a biaxially stretched film which is decomposable in the natural environment, and by solving the above problems, allows tacking, has heat stickability, suppresses thickness unevenness, break, whitening and unevenness, and which has stretching stability.
In order to solve the above problems, the present invention provides a biodegradable biaxially stretched film comprising a polylatic acid-family polymer as the major component, and having a storage elastic modulus at 120xc2x0 C. of 100-230 MPa as measured using test method concerning temperature dependency of the dynamic viscous elasticity under JIS K7198.
Preferable embodiments of this invention include a biodegradable biaxially stretched film having an area stretching ratio of 6.8 times or over, biaxially stretched at a longitudinal stretching temperature of 70-90xc2x0 C. and a lateral stretching temperature of 70-80xc2x0 C., and after biaxial stretching, heat-set at a temperature of 100xc2x0 C. to melting point (Tm) in a gripped state, a biodegradable biaxially stretched film having an area stretching ratio of 6.8 times or over, simultaneously biaxially stretched at a stretching temperature of 70-80 xc2x0 C., and after biaxial stretching, heat-set at a temperature of 100xc2x0 C. to melting point (Tm) in a gripped state, a biodegradable biaxially stretched film having a tensile strength of 1000-2000 kgf/cm2 and a tensile elongation of 5-150% as measured at a tensile speed of 200 mm/min using a No. 2 test piece under JIS K 7129, and a biodegradable biaxially stretched film having the weight-average molecular weight of the polylactic acid-family polymer of 60000-700000.
Hereinbelow, embodiments of this invention will be described.
The biodegradable biaxially stretched film according to this invention is a film comprising a polylactic acid-family polymer as its major component and having a storage elastic modulus Exe2x80x2 at 120xc2x0 C. of 100-230 MPa.
The polylactic acid-family polymer is a homopolymer of D-lactic acid or L-lactic acid, or a copolymer of D-lactic acid or L-lactic acid. It may contain other hydroxy-carboxylic acid units as a small amount of copolymeric components and may also contain a small amount of chain extender residual groups.
As the polymerizing method, a known method such as condensation polymerization or ring opening polymerization may be used. For example, in the condensation polymerization, it is possible to obtain a polylactic acid having any desired composition by directly subjecting L-lactic acid, D-lactic acid or mixture thereof to dehydration condensation polymerization.
Also, in the ring-open polymerization method (lactide method), by polymerizing a lactide, which is a cyclic dimer of a lactic acid, it is possible to obtain a polylactic acid using a selected catalyst and a polymerization adjusting agent or the like as necessary.
The weight-average molecular weight of the polylactic acid-family polymer is preferably 60000-700000, more preferably 80000-400000, and most preferably 100000-300000. If the molecular weight is less than 60000, practical physical properties such as mechanical properties and heat resistance will scarcely reveal. If higher than 700000, the melt viscosity will be too high, so that molding workability is poor.
As other hydroxy-carboxylic acids as the small-amount copolymer components, it is possible to cite optical isomers of lactic acids (D-lactic acid for L-lactic acid and L-lactic acid for D-lactic acid), 2-functional aliphatic hydroxy carboxylic acids such as glycolic acid, 3-hydroxy butyric acid, 4-hydroxy butyric acid, 2-hydroxy-n-butyric acid, 2-hydroxy-3,3-dimethyl butyric acid, 2-hydroxy-3-methyl butyric acid, 2-methyl lactic acid, and 2-hydroxy caproic acid: lactones such as caprolactone, butyrolactone and valerolactone.
Further, if necessary, as a small amount of copolymerizing component, nonaliphatic dicarboxylic acids such as terephtahlic acid and/or nonaliphatic diols such as ethylene oxide adducts of bisphenol A may be used.
For the purpose of adjusting various physical properties, heat stabilizers, light stabilizers, light absorbers, lubricants, plasticizers, inorganic fillers, colorants, pigments, etc. may be added.
As a method of manufacturing a biaxially stretched film of which the major component is a polylactic acid-family polymer, a method may be used in which after a web-like product or a cylindrical product extruded from a T-die, I-die or round die has been solidified in a state close to amorphous by quenching it by use of cooling cast rolls, water or pressurized air, it is biaxially stretched by the roll method, tenter method, tubular method, etc.
For the manufacture of a biaxially stretched film, successive biaxial stretching method in which longitudinal stretching is carried out by the roll method and followed by lateral stretching by the tenter method, or simultaneous biaxial stretching method in which longitudinal and lateral stretching are simultaneously carried out by use of a tenter is normally used.
Stretching conditions may be selected within the range of 1.5-6 times in the longitudinal direction and 1.5-6 times in the lateral direction. Further, in view of the film strength and evenness of thickness, stretching is preferably two times or over both in longitudinal and lateral directions. In particular, the area stretching ratio obtained by multiplying the longitudinal and lateral stretching ratio is preferably 6.8-36 times.
In the successive biaxially stretching method, the longitudinal stretching temperature is preferably 70-90xc2x0 C. and the lateral stretching temperature is preferably 70-80xc2x0 C. In the simultaneous biaxially stretching method, because it is included in the successive biaxially stretching method, stretching is preferably carried out at the stretching temperature of 70-80xc2x0 C. If the area stretching magnification and the stretching temperature are not within the abovesaid ranges, the evenness of thickness of the film obtained tends to be extremely low. This tendency is especially remarkable with a film that is heat-set after stretching. Such an unevenness in thickness is a factor that can severely cause wrinkles or corrugation in the secondary working such as printing on films, lamination on other films or on metallic foil or paper, or bag making.
In order to suppress heat shrinkage of film, after biaxial stretching, it is important to carry out heat setting with the film gripped. Normally in the tenter method, since film is stretched while being gripped by a clip, it is possible to carry out heat setting immediately after stretching.
The heat setting temperature is, though depending upon the melting point of the polylatic acid-family polymer used, preferably in the range of 100xc2x0 C. to melting point (Tm). The heat setting time is preferably at least 3 seconds. If it is below such a range, the heat shrinkage rate of the film obtained is too high, so that problems such as shrinkage of film tend to occur in the secondary working of the film. In order not to cause such problems, it is important that the heat shrinkage is 5% or less in warm water 80xc2x0 C./10 seconds, preferably 3% or less. If the heat setting temperature is over the melting point, the film will melt during heat setting, which will break the film.
The biodegradable biaxially stretched film obtained by such stretching and heat setting should have a storage elastic modulus Exe2x80x2 at 120xc2x0 C. of preferably 100-230 MPa, more preferably 120-200 MPa as measured using the test method concerning temperature dependency of dynamic viscous elasticity under JIS K 7198. If the Exe2x80x2 value is larger than 230 MPa, the crystallinity of the film would be too high, which will lower the content of the amorphous portion. This lowers heat-stickability of the film, making it difficult to finish with the film to a beautiful package. But since the shrinkage of the film is low, when a hot plate is applied against it, the film will not shrink. If the Exe2x80x2 value is smaller than 100 MPa, the shrinkage of the film is too high, so that even though it has heat-stickability, when a hot plate is pressed against it, finish will be poor in appearance. Further, the secondary workability will be inferior too.
Specifically, in bag making using a heat setting machine in which film is heated by burning and cutting it with heat ray, uneven shrinkage occurs at heat-set portions due to heat transfer, thus worsening the finish.
The biodegradable biaxially stretched film obtained by such stretching and heat setting has preferably a tensile strength of 1000-2000 kgf/cm2 and a tensile elongation of 50-150%, more preferably a tensile strength of 1100-1500 kgf/cm2 and a tensile elongation of 60-120%, as measured at a tensile speed of 200 mm/min using No. 2 test pieces under JIS K 7127. If the tensile strength is less than 1000 kgf/cm2, in the second working such as printing or bag making, when the film is unrolled while longitudinally applying tension, if it is thin, the film may not withstand the tension and break. Also, if it is over 2000 kgf/cm2, in laminating the film with another film, metallic foil or paper, due to tension applied during working, stress may remain in the finished laminate and it curls. If the tensile elongation is less than 50%, as with the tensile strength, it tends to break during secondary working. If it is over 150%, the film will be insufficient in elastic deformation. If tension is applied during secondary working, the film may be plastically deformed, so that sagging tends to occur in the film, which can cause wrinkles in the film.
The biodegradable biaxially stretched film according to this invention can be used for folded packaging for video tapes, cassette tapes, compact discs, floppy discs, etc. and for folded packaging for tobacco, caramel and granular gum.